Diaphragm pumps are used in many pumping applications, and offer several distinct advantages as compared to rotary and other types of pumps. Diaphragm pumps have good suction lift characteristics, good dry running characteristics, and can be up to 97% efficient. Various types of diaphragm pump work well with air and with highly viscous liquids, and can have good self-priming capabilities. Depending on the design, diaphragm pumps can also minimize the number of moving parts that are in contact with the process fluid. This can be ideal for applications to gritty and/or highly viscous liquids, and to corrosive liquids and gases.
FIG. 1 is a cross-sectional illustration of a diaphragm pump of the prior art. It is described in more detail in co-pending application Ser. No. 14/332,627, which is incorporated herein by reference in its entirety for all purposes. The pump includes a pump housing 118 that surrounds a pumping chamber 100 having a fluid inlet 102 and a fluid outlet 104. The pumping chamber 100 is bounded on one side by a flexible diaphragm 106, which can be distorted so as to increase and/or decrease the volume of the pumping chamber 100. One or more inlet and outlet valve assemblies 110 control the flow of process fluid, so that when the volume of the pumping chamber 100 is increased, process fluid is drawn into the pumping chamber 100 through the fluid inlet 102 and through the inlet valve assemblies 110, and when the volume of the pumping chamber 100 is decreased, process fluid flows out of the pumping chamber 100 through the outlet valve assemblies 110 and into the outlet.
In theory, there should be no leakage past the inlet and outlet valve assemblies 110 when they are closed. However, in practice some leakage can occur. Leakage can be especially problematic when pumping a compressible gas, for example because the fluid to be pumped is a gas, or during self-priming as a liquid is first drawn into the unprimed pump. Of course, any such valve leakage will significantly reduce the efficiency of the pump.
With reference to FIGS. 2A and 2B, in many diaphragm pumps, including the example illustrated in FIG. 1, each of the inlet and outlet valve assemblies 110 includes a seat 200 having a central shank 204 into which a stem 200 of a valve 206 manufactured from an elastomer is inserted. The seat also includes a sealing surface 218 against which a valve sealing surface 220 of a flexible extension 222 of the valve 206 is pressed when the valve 206 is installed in the seat 200. Both the valve 206 and the valve seat 200 are typically symmetric about a central, vertical axis 208. The seat 200 may be formed in a valve plate 116, and the sealing surface 218 of the seat 200 will typically include an inner region 210 in which an inner part 214 of the flexible extension 222 of the valve 206 is supported by a plurality of spaced-apart support beams (see FIG. 6) that allow fluid to pass through the valve plate 116 when the valve assembly 110 is open. The sealing surface 218 of the seat 200 will typically further include an outer sealing region 212 that forms a fluid seal with a corresponding outer region 216 of the valve sealing surface 220 when the valve assembly 110 is closed.
The stem 202 of the valve 206 will typically be slightly shorter than the shank 204, so that the valve sealing surface 220 will be pressed against the sealing surface 218 of the seat 200 when the valve assembly 110 is closed. Often, the valve 206 will be shaped such that at least some portion of the valve flexible extension 222 will be deflected upward by the sealing surface 218 of the seat 200 when the valve stem 202 is installed in the shank 204.
The tension between the outer region 216 of the valve sealing surface 220 and the sealing region 212 of the valve sealing surface 220 when the valve assembly 110 is closed will depend, among other factors, on the relative lengths of the stem 202 and shank 204, the elasticity of the valve flexible extension 222, and the relative shapes of the valve flexible extension 222 and the sealing surface 218 of the valve seat 200. This tension must be adjusted to a value that is essentially a compromise. Too much tension in the inner region 210 will inhibit the valve from fully opening. Too little tension in the outer region 212 will increase the likelihood that the valve assembly 110 will leak when nominally shut.
Adjusting the valve tension is made even more problematic due to a “leverage” effect, as illustrated in FIG. 2B. For any given force applied by the stem 202 and shank 204 to the valve flexible extension 222, relatively more force will be applied near the center 208 of the valve 206, indicated by letter “A” in the figure, while relatively less force will be applied further from the center 208, e.g. near the rim as illustrated by letter “B” in the figure, due to the flexibility of the valve flexible extension 222 and the greater leverage distance between the outer region 216 and the center 208. Accordingly, it can be difficult to find a compromise solution that will allow adequate flow through the valve assembly 110 when it is open, but will also minimize leakage when it is shut.
One approach is to rely on the pressure applied by a process fluid to the outer region 216 of the valve flexible extension 222 to minimize leakage. However, process fluid pressure cannot always be relied upon to hold the valve 110 shut, especially when a compressible gas is being pumped through the valve 110.
What is needed, therefore, is a valve assembly that reduces leakage when closed while providing good flow-through when open.